1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, printer or similar image forming apparatus and more particularly to an image forming apparatus of the type using a revolver or rotary developing device.
2. Description of the Background Art
An image forming apparatus of the type using a revolver as a developing device is conventional. The revolver includes a rotary developing unit adjoining an image carrier and having a plurality of developing sections arranged therein. A toner storing unit is rotatable integrally with the developing unit and has a plurality of toner chambers corresponding one-to-one to the developing sections of the developing unit. A plurality of toner conveying means each connect one toner chamber to the associated developing section. This type of image forming apparatus is disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 62-251772 and 63-78170 and Japanese Utility Model Laid-Open Publication No. 63-41164. Each toner conveying means includes a toner passage extending between the toner chamber and the developing section and a toner conveying member disposed in the toner passage. The toner conveying means conveys fresh toner from the toner chamber to the developing section, as needed.
As for the toner conveying member, Japanese Utility Model Laid-Open Publication No 3-129968 and Japanese Patent Application No. 4-345373, for example, each teach a screw made up of a shaft and a spiral blade formed on the shaft. Such a toner conveying member is capable of conveying toner from a toner chamber to a remote place along, e.g., a toner pipe. This enhances the free layout of the individual unit of the image forming apparatus, e.g., allows each toner chamber to be positioned not on the top, but on the side of the associated developing section, while contributing to the miniaturization of the apparatus.
A certain clearance is formed between the screw and the toner pipe, so that the screw can smoothly rotate. If the clearance is absent, then the screw and toner pipe are apt to produce noise due to interference or lock up when the screw is driven to rotate. The clearance, however, brings about the following problems.
First, the toner whose fluidity increases due to the rotation of the revolver rushes into the toner pipe and flows into the developing section via the clearance. Second, even when the fluidity of the toner is not high, the toner rushed into the toner pipe makes it difficult for air inside the toner pipe to escape. The resulting air pressure forces the toner out of the toner pipe toward the developing section side. Third, part of the toner adjoining a toner outlet, which is formed in the toner pipe and faces the developing section, flows into the developing section due to vibration ascribable to the rotation of the revolver. In any case, the toner unexpectedly flown out of the toner pipe enters the developing section without regard to a toner replenish signal to be output from the apparatus body. If the toner flows out of the toner pipe in an amount greater than one to be consumed, then it increases the toner content of a developer stored in the developing section, resulting in excessively high image density and background contamination. Moreover, such toner increases the volume of toner present in the developing section and causes the developing section to lock up.
In light of the above, Japanese Patent Laid-Open Publication No. 2000-56568, for example, discloses an image forming apparatus including a revolver operable with a magnetic developer. A plurality of toner cartridges are removably mounted to the revolver in one-to-one correspondence to developing sections arranged in the revolver. The toner cartridges are rotatable integrally with the revolver. A plurality of toner conveying means each include a toner passage connecting one of the toner cartridges to the associated developing section and a toner conveying member. Magnetic field forming means is provided on the toner passage in the vicinity of a toner outlet, which is formed in the toner passage and communicated to the developing section. The magnetic field forming means forms a magnetic field that causes the magnetic developer, which flows out of the developing section into the toner passage when the revolver rotates, to form a magnet brush around the toner outlet. The magnet brush seals the toner outlet to thereby prevent the toner from flowing into the developing section. The magnet brush, however, cannot fully seal the toner outlet.
Generally, the rotation speed of the revolver is varied in accordance with the number of images to be formed. In a high-speed machine, for example, the revolver is caused to rotate at a higher speed than in an ordinary machine. The rotation speed of the revolver presumably has influence on the amount of toner to unexpectedly flow out of the toner passage into the developing section. Further, the current trend in the imaging art is toward the use of toner having high fluidity, i.e., low cohesiveness for enhancing image quality. Toner with low cohesiveness is rapidly mixed with a developer and rapidly stabilizes the characteristics of the developer. Presumably, the cohesiveness of toner also has influence on the amount of toner to unexpectedly flow into the developing section.
It is therefore necessary to clear up the influence of the rotation speed of the revolver or the cohesiveness of toner on the amount of toner to unexpectedly flow into the developing section to thereby obviate the unexpected outflow of the toner. It is also necessary to promote accurate prevention of the unexpected outflow of the toner.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-194947, 6-236112 and 2000-250314 and Japanese Patent No. 2,935,114.
It is an object of the present invention to provide an image forming apparatus capable of preventing toner from unexpectedly flowing out of a toner cartridge into a developing section and thereby insuring high image quality.
An image forming apparatus of the present invention includes an image carrier. A rotary developing device adjoins the image carrier and includes a plurality of developing sections for developing a latent image formed on the image carrier with a magnetic developer. A toner cartridge unit includes a plurality of toner cartridges and is rotatable integrally with the developing device. A plurality of toner conveying device each connect one toner cartridge to the associated developing section and each include a toner passage and a toner conveying member disposed in the toner passage. A magnet adjoins a toner outlet, which is formed in the toner passage and communicated to the developing section, for forming a magnetic field that traverses the toner passage over the entire diameter of the toner passage. Toner stored in each toner cartridge is prevented from unexpectedly flowing into the associated developing section when replenishment is not effected.